1. Field of the Invention
The invention relates to a method of obtaining images of a nuclear magnetization distribution in a part of a body, in which nuclear spins are excited in the body in the presence of a steady magnetic field in order to generate a spin resonance signal, after which the spin resonance signal is sampled in the presence of a magnetic field gradient which is superposed on the steady magnetic field, after which the resonance signals thus sampled are converted into an image of said distribution by signal transformation. The sampled resonance signals may be pulse echo signals or field echo signals according to the described method. The invention also relates to an MRI device comprising a magnet system for generating a steady, uniform magnetic field and for generating magnetic field gradients, and also comprising an RF transmitter and receiver for generating and detecting spin resonance signals, a sampling device for sampling the resonance signals generated, a signal transformation device for transforming the sampled spin resonance signals, a display device for displaying transformed signals, and a control device for controlling the magnet system, the RF transmitter and receiver, the sampling device, the signal transformation device and the display device.
2. Prior Art
Such a method and device are known from U.S. Pat. No. 4,070,611 and are often used for obtaining in vivo images on the basis of the density and other properties of the nuclear magnetization distribution in the object examined. Furthermore, PCT Application WO 85/02264, which corresponds to U.S. Pat. Nos. 4,573,014 and 4,634,979, describes a method of synthesizing such images of an object or a part of an object. Therein, it is assumed that the signal intensity in a point of an image is determined by the local magnetization MO, the local longitudinal and transverse relaxation time constants T1 and T2, and the parameters used in the measuring method, for example the repetition time of the measuring cycles TR and the waiting time TE elapsing between the excitation at the beginning of the measuring cycle and the instant of sampling of the spin resonance signal. By determining a number of images where the measuring parameter TR as well as the measuring parameter TE is varied, the magnetization MO and the longitudinal and transverse relaxation time constants T1 and T2 can be calculated for each pixel. Once this data is known, for arbitrary TR and TE synthetic images can be reconstructed as if these images had been obtained during a measurement characterized by these measuring parameters TR and TE. Thus, images can be synthesized in which the contrast between the various parts of the image can be optimized. The described method thus always requires a number of complete images before synthetic images can be reconstructed. This has direct consequences for the measuring time required, i.e. for the time during which an object must remain in the MRI device.